The functions of the bronchiolar Clara cell are not known although it is generally believed that the cell is secretory. Using Clara cells isolated from the lungs of rabbits we have shown that the major protein secreted by bronchilar Clara cells is a low molecular weight protein immunochemically and compositionally similar to uteroglobin, the major secretory protein of the uterine epithelium. Rabbit tracheal cells also secrete uteroglobin although morphologically there are no cells in the tracheal epithelium identical to bronchiolar Clara cells. We have identified tracheal epithelial cells containing uterglobin by using an immunogold procedure. These investigations reveal that uterglobin-containing tracheal cells, like those of the bronchioles are nonciliated and contain abundant smooth endoplasmic reticulum. However, unlike the bronchiolar Clara cell, the tracheal cell is tall and columnar in overall shape and it contains few secretory granules.